A typical brushless DC motor has permanent magnets which rotate and a fixed armature, eliminating problems associated with connecting current to a moving armature as with brushed DC motors. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system.
Starting up a three-phase sensorless brushless DC (BLDC) motor based on a phase-center tap motor configuration is one of the major concerns for driver and controller design. For instance, problems with low back-electromotive force (BEMF) voltages at low speed can occur where it may not be reliable to use BEMF zero crossing for commutation, for example. To actuate the motor, one common prior technique utilizes open loop control at the start of motor operations until the BEMF voltage is adequate. A large variation on load inertia however could exceed the capability that open loop controls can manage and thus, leading to start-up failure. When pulse width modulation (PWM) is involved during start up, mutual inductance effects in the motor can mask BEMF voltages thus causing improper commutation and start up failure.